To deal with emergencies or perform maintenance service in the operation of liquid sodium or sodium-potassium metal cooled nuclear reactors for power generation it may be necessary to shut down the fission reaction of the fuel. Normally reactor shut down is achieved by inserting neutron absorbing controls into the core of fissioning fuel material to deprive the fuel material of neutrons needed to perpetuate the fission reaction. However decay of the fuel material in the shut down nuclear reactor continues to produce heat in substantial quantities which must be continuously dissipated from the reactor system.
The heat capacity of the liquid metal coolant and adjacent structural material assist in dissipating the residual heat. Nevertheless, the structural material of the nuclear reactor may not have the capacity of safely withstanding prolonged high temperatures. For example, the concrete of the walls of the typical reactor silo and other structures may splay and crack when subjected to high or prolonged raised temperatures. Accordingly, auxiliary cooling means or systems are commonly utilized to safely remove heat from the nuclear reactor structure during periods of reactor shut down.
Conventional nuclear reactors have employed a variety of complex energy driven cooling measures to dissipate heat from the fuel core and other components of the reactor system. Occasionally when a reactor shut down occurs, the energy source for actuating and/or operating such auxiliary cooling means may fail. For example, pumps and ventilation systems for performing supplementary fuel core cooling may malfunction or lack power. Moreover, when operator personnel intervention is needed, there are potential situations when an operator may not be responsive or be capable of performing the required action.
Liquid metal cooled reactors utilizing sodium or sodium-potassium as the coolant provides numerous advantages. Water cooled reactors operate at or near the boiling point of water. Any significant rise in temperature results in the generation of steam and increased pressure. By contrast, sodium or sodium-potassium has an extremely high boiling point, in the range of 1800 degrees Fahrenheit at one atmosphere pressure. The normal operating temperature of the reactor is in the range of about 900 degrees Fahrenheit. Because of the high boiling point of the liquid metal, the pressure problems associated with water cooled reactors and the steam generated thereby are eliminated. The heat capacity of the liquid metal permits the sodium or sodiumpotassium to be heated several hundred degrees Fahrenheit without danger of materials failure in the reactor.
The reactor vessels for pool-type liquid-metal cooled reactors are essentially open top cylindrical tanks without any perforations to interrupt the integrity of the vessel walls. Sealing of side and bottom walls is essential to prevent the leakage of liquid metal from the primary vessel. The vessel outer surfaces must also be accessible for the rigorous inspections required by safety considerations.
Upon shutdown of the reactor by fully inserting the control rods, residual heat continues to be produced and dissipated according to the heat capacity of the plant. Assuming that the reactor has been at full power for a long period of time, during the first hour following shutdown, an average of about 2% of full power continues to be generated. The residual heat produced continues to decay with time.
To maximize the power capacity in liquid metal cooled, pool type nuclear fission reactor plants, such as noted above and disclosed in the cited patents, it has been proposed to locate the reactor coolant circulating pump and primary heat exchanging units outside of the reactor vessel pool. This system enables the utilization of a larger heat producing fuel core within the reactor vessel, or a reduction in the size of the reactor vessel, which returns certain benefits. Liquid metal cooled pool nuclear reactors of this type comprise multiple component vessels, including the fuel containing reactor primary vessel, assembled with external satellites of one or more circulating pump units and heat exchanger units, operatively connected by top entry loops or conduits for coolant circulation in series between each separate component vessel.
This top entry loop joined, "satellite" assembly reactor system comprises a reactor primary vessel containing a core of fissionable fuel submerged in liquid metal coolant and at least one primary heat transferring liquid metal coolant circuit or loop including a coolant circulating pump component housed within a separate vessel and a heat exchanger component housed within another separate vessel. Top entry loop conduits connect each component vessel in series to provide for liquid metal coolant circulation from the fuel core containing reactor vessel to the pump vessel, then to the heat exchanger vessel and finally back into the reactor vessel to repeat the cycle continuously during operation for transferring fuel core produced heat to the heat exchanges.
This type of top entry loop, multiple component and vessel, satellite reactor system is illustrated in an article entitled "Cost Reduction Study Of A 1000MWe Loop-Type Demonstration Fast Breeder Reactor" by H. Nakagawa et al, published in the Proceedings of the International Conference On Fast Breeder Systems: Experience Gained and Path to Economic Power Generation. Sept. 13-17, 1987, pages 4.10-1 to 4.10-11.
Liquid metal cooled pool reactors of such a satellite arrangement comprise open top unit vessels which are closed and protected by means of a shield deck which extends across the open upper end of the reactor vessel and any associated units contained in vessels. Commonly a single shield deck structure bridges the entire expanse of the upper end of the complete assembly of vessels of the satellite system, and may include extending over the open top of the concrete silo.
The invention comprises an improvement in a passive cooling means for removing shutdown decay heat from top entry loop liquid metal coolant, pool nuclear reactor plants.
The disclosed contents of the above noted U.S. Pat. Nos. 4,508,677, 4,678,626, and 4,959,193, and the cited article, all comprising background art, are incorporated herein by reference.